1. Field of the Invention
The present invention relates to processes and reagents for inhibiting or reducing scale formation in and/or on process equipment throughout various stages of phosphoric acid production.
2. Description of the Related Art
The wet process is the most commonly used process in phosphoric acid production. In the wet process, phosphate rocks, which contain mostly calcium phosphate, are cleaned in a wash plant and ground in a Ball mill before being fed into a series of reactors for digestion with sulfuric acid along with recycled phosphoric acid from the process. The digestion temperature typically ranges from 40° C. to 80° C. After completing the reaction series, the process stream is washed with evaporator condensate while being forced through a filter.
After digestion, the reaction slurry is filtered to separate phosphoric acid from Gypsum (calcium sulfate). The filtered crude phosphoric acid is then sent to the clarifiers and the evaporators for further purification and concentration. The purified phosphoric acid is either sent out as 28% Merchant Grade Acid (MGA) or continued to make 69% P2O5Super Phosphoric Acid (SPA). The Gypsum is washed and dried before being sold for commercial uses. Some of the crude phosphoric acid is concentrated to 44% (P2O5) before sent for Monoammonium Phosphate (MAP), Diammonium Phosphate (DAP) and ammonium phosphate-sulfate (APS) production.
Due to the supersaturated nature of the acid and the impurities in the phosphate ores, the concentration steps with respect to P2O5 render several side reactions, causing scale formation at different stages of the phosphoric acid production. For example, fluorosilicate is one of the more common scale species found in phosphoric acid production. It can be depicted by the following equations:

More than 12-15 other types of scaling species can be found throughout phosphoric acid production and they have provided significant challenges for the industry. Plants producing phosphoric acid normally have to shut down production every few weeks to physically clean off the scales either using high pressure water or other mechanical means. The economic impact for the scale-related issues is substantial, and the industry is in need of a more efficient scale prevention technology than the existing physical means of post scale formation removal.
Conceptually, there are two basic types of approaches scale removal from the phosphoric acid production process—namely, the physical method and the chemical method. There are several options for the physical method. In addition to the previously mentioned mechanical and water wash method, magnetic separation (Wang, Chuhua; Benson, Robert F.; Martin, Dean F. Enhanced solubility of sodium fluorosilicate scale by magnetic treatment, Florida Scientist (1998), 61(1), 17-25) and ultrasonic methods (Pandey, A. D.; Mallick, K. K.; Pandey, P. C.; Varma, S. Prevention of scale deposition on heat exchanger surfaces by use of high intensity ultrasonic waves during concentration of wet process phosphoric acid, Fertiliser News (1983), 28(6), 45-8) have also been used as part of the physical approach. Another approach still, is available by using physically smoothed piping in phosphoric acid production (See DE 3039187). Among all these options, chemical treatment methods for scale inhibition appear to be more practical and efficient. Typically, chemical methods require limited amounts of capital investment and have the potential not to alter the existing process in the phosphoric acid plants. Processes and reagents that do not require large amounts of reagent and therefore have minimal environmental and downstream impact are also preferable.
Although there have been numerous attempts to address the scale problem in boiler water systems (for example, copolymers of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) were reported to reduce the amount silica gel adhering to the wall of the testing bottles in EP0271035. These polymers were reported to reduce the amount of silica gel adhering to the wall of the testing bottles. Other systems such as polyamine, phosphonic acid and carboxylic acid based monomers and polymers have also been reported to show effectiveness in scale removal in boiler water system (for examples, see GB2424876, JP2002263690, EP0677485), the environment found in boiler water systems differs vastly from that found in the wet phosphoric acid production. The boiler water systems typically have mild conditions with pH in the range of 8 to 9 and low concentrations of dissolved salts. In direct contrast, the wet phosphoric acid process normally contains harsh condition with low pH and high solids content. In addition, the scale formed in phosphoric acid plants has much more complicated components—more than 15 known species, such as Na2SiF6, K2SiF6, CaSiF6.2H2O, CaF2, MgF2, CaSO4.2H2O (Gypsum), MgSiF6.6H2O, Mg0.8Al1.5F6.XH2O, MgH2P6O7, CaSO4, Al(PO3)3, NaK2AlF6, Ca3(AlF6)2.4H2O, MgNaAlF6.2H2O, Ca4SO4AlSiF13.10H2O (see for example, A. William Frazier, James R. Lehr, and Ewell F. Dillard, Environmental Science 8. Technology, 11, 1007, 1977). Moreover, different phosphoric acid plants experience different types of scales and even within one plant, the type of scale can differ greatly from one location to the other. With such a complicated scale system, it becomes a great challenge to develop scale inhibition reagents for phosphoric acid plants.
Not surprisingly, there is very little information addressing the phosphoric acid plant scale issue in an industrial setting. Even in the academic context, the results are scattered. For example, several articles mention reagents for fluorosilicate inhibition in phosphoric acid production. (see for example, L. Yang, Zhang Y., Huang, Y. Chemical Industry and Engineering (China), (2002), V 19(1), 1). A Chinese patent (CN1762857) reports that mixtures of copolymers such as polyacrylic acid and polymaleic acid, polysulfonates, plus phosphonates and a tetraalkyl ammonium chloride combination reduces scale formation in wet process phosphoric acid production. A US patent (U.S. Pat. No. 5,120,519) teaches that high molecular weight polyacrylamide and polyacrylic acid can prevent scale from adhering on the surface of the phosphate rock and phosphoric acid. However, the use of most these chemicals are not new and have been applied water treatment systems for scale control and the mechanism of these reagents is based mostly on their dispersant effect.
Accordingly, the compositions and methods presently available for preventing and/or reducing scale in the phosphoric acid production process require further improvement. Compositions and formulations that effectively prevent and/or reduce scale, thereby enabling the phosphoric acid production plant to run longer without shutting down to remove scale would be a useful advance in the art and could find rapid acceptance in the industry.